In aeronautics, it is known that the noise produced by an aircraft, such as an airplane, is particularly unpleasant and unbearable for people in the region of an airport, i.e. those living in dwellings close to an airport. This noise is particularly intense during the take-off phases of aircraft. Hence, for many years, aircraft manufacturers been trying to attenuate the noise produced by aircraft to the utmost, especially during take-off.
As a rule, the noise caused by an aircraft turbofan engine, especially under full power, is generated by the speed of ejection of burnt gases at the exhaust outlet. From an installation viewpoint, the turbofan engine is housed in a nacelle equipped in its rear part with primary and secondary exhaust systems. The primary conduit, corresponding to the primary exhaust system, has a primary nozzle whose purpose is to convey a stream of hot air into the open at constant energy of speed. The secondary conduit, corresponding to the secondary exhaust system, is defined by the volume between the external part of the primary nozzle and the internal part of the thrust inverter. Air exhaust systems, on the whole, have the function of converting the pressure energy of the gases into energy of speed and hence into thrust. The speed of air at the outlet from the primary nozzle prompts a jet noise. The higher the engine speed, the greater is this jet noise. Since an aircraft has its maximum thrust at take-off, it will be understood that the jet noise caused by its engines is the maximum at take-off. Once the aircraft has reached its cruising phase, the noise caused by the engines is often less important, because of the distance of the aircraft from dwellings, and especially because of its high altitude.
At present, to attenuate the jet noise from the turbofan engines of an aircraft during take-off, there is a prior art method of mounting primary exhaust nozzles with fixed chevrons to the aircraft. A chevron exhaust nozzle is a nozzle whose external end, i.e. the end that forms the trailing edge of the nozzle, has a non-smooth shape forming “zigzags”. These zigzags may be constituted by a succession of Vs. A chevron exhaust nozzle may be a double-coated nozzle, i.e. a nozzle with an inner coat through which there passes the flow of hot air (the primary flow) and an outer coat in contact with a flow of cold air (secondary flow). A chevron nozzle of this kind has the effect of more efficiently homogenizing the primary and secondary flows and reducing the sonic nuisance caused by the air at high speed in the primary flow.
However, a chevron nozzle has a negative influence on the performance of the engine. Indeed, acoustic optimization lowers the efficiency of the nozzle in terms of pure performance. For equal thrust, a turbofan engine with chevron exhaust nozzles consumes a substantially greater quantity of fuel than a turbofan with smooth nozzles. Indeed, turbulence is then generated, thus inducing load losses and, therefore, greater fuel consumption. In small-sized aircraft that make short flights, the effect of the increase in this specific consumption is small (it is about 0.1%) since the cruising phase is short. However, in large aircraft making long-haul flights, fuel consumption, which is basically substantially greater than in a more modestly sized aircraft, rises significantly. Indeed, it will be understood that the greater the duration of the flight, the more appreciable will be the increase in consumption, and this has a well-known economic effect.
Furthermore, at low speed, the presence of chevron exhaust nozzles increases the thrust of the aircraft whereas, at high speed, the thrust, for a given motor speed, is affected negatively. Hence, to avoid modifying the performance of the aircraft, it is possible to set up an automatic control link between an adjustment of thrust and the speed of the aircraft. The defining of such an adjustment is complex and can prove to be costly. Furthermore, a surveillance system has to be implemented to ensure the efficient configuration of the nozzle.